The present invention relates to a method and an apparatus for sharpening tools and, more particularly, to a method and an apparatus to aid in sharpening a tool by facilitating proper positioning of an abrasive surface relative to the bevel face defining the tool's cutting edge.
Cutting tools, and particularly, those used in woodworking, such as chisels, plane irons, and the like, frequently require sharpening or honing and, to some extent, may even require grinding. Many of these tools comprise a blade with a cutting edge defined by a flat bevel face that opposes another major face at an edge of the blade. These tools are typically sharpened by manually holding the cutting tool and bringing it into contact with a rotating abrasive element, such as a grinding wheel, or otherwise, by manually moving the cutting tool against a fixed abrasive element such as a sharpening stone.
Manufacturers of cutting tools have the necessary, and often expensive, equipment to make tool sharpening a relatively simple task. However, this expensive and technically advanced sharpening machinery is typically not available to users of these cutting tools making it difficult for a user to obtain a precise cutting edge when sharpening the tool. In order to achieve proper cutting with a tool, it is necessary to maintain a uniform bevel at the cutting edge. Moreover, the uniformity of the bevel is desirably duplicated on each occasion when the tool is sharpened. When a tool is hand-held and brought into contact with an abrasive element, or when the abrasive element is hand-held and brought into contact with the tool by some manual means, it is difficult to maintain precise alignment of the blade of the tool and the abrasive element and uniformity in the cutting edge bevel is materially affected.
There are devices which function as guides for sharpening chisels and plane irons. These devices typically comprise a frame that holds the chisel or plane iron at an angle to a fixed abrasive stone. The frame includes a roller or other support at one end that rides on a table, the abrasive stone, or another supporting surface while the chisel or plane blade is clamped to the other end of the frame and extends into contact with the abrasive stone. One of the purposes of these devices is to achieve a precise angle on the cutting edge bevel. However, when manually moving a tool in contact with an abrasive stone, it is easy to rock the tool and difficult to apply uniform pressure on the bevel across the width of the blade and, therefore, easy to skew the cutting edge relative to the other surfaces of the blade.
Conventional bench grinding wheels, typically comprising an abrasive stone rotated by an electric motor, are often used for sharpening tools. The tool is physically held in contact with the rotating abrasive stone. However, it is difficult to keep the tool at the correct angle and achieving a flat bevel face is difficult when the grinding is performed by the curved surface of the perimeter of the grinding wheel.
Laughton, U.S. Pat. No. 4,996,797 discloses a tool sharpening apparatus comprising a grindstone rotated about an axis by a motor and a tool rest rotatable about an axis parallel to the axis of rotation of the grindstone. The tool rest includes a surface arranged at an angle to a face of the grindstone that is normal to the stone's axis of rotation. To sharpen a tool, the blade is placed against the angled tool rest surface, pushed parallel to the surface of the tool rest until the bevel face contacts the stone, and then the tool rest is rotated to sweep the end of the tool in an arc across the face of the grindstone. While the bevel is in contact with a planar abrasive surface, moving the end of blade in an arc will produce a cutting edge with a convex curve.
In another specialized powered sharpening system, the user clamps the tool in a tool holder having a curved surface corresponding to a curved surface on a tool rest arranged above an abrasive surface of a powered disk. The user can rotate the clamp and tool about the curved surface of the tool rest to bring the bevel face into contact with the moving abrasive surface. The disk provides a planar abrasive surface, as opposed to the curved surface of the perimeter of a grinding wheel, promoting a planar surface for the cutting edge bevel. On the other hand, care must be taken in adjusting the height of the tool rest and the projection of the blade in the tool holder so that when the cutting edge of the tool rotates around the tool rest, the bevel face will contact the abrasive surface at the correct angle.
While an experienced craftsperson can sharpen a cutting tool quite accurately, many users of cutting tools are not experienced craftspersons. Holding a cutting tool and bringing it into contact with an abrasive element is an imprecise way to grind, sharpen, or hone a tool. Usually, a person holding a tool or an abrasive element or moving one relative to the other, cannot hold and move the tool or abrasive element with the uniformity of pressure and angle of attack necessary to achieve a linear cut which is uniform across the length of the cutting edge of the tool. While aids for powered tool sharpening are available, they are typically relatively expensive and require considerable skill and experience to achieve optimum results.
What is desired, therefore, is an inexpensive and effective apparatus that allows an inexperienced user to reliably and correctly sharpen a cutting tool by fixing the bevel face of the blade of the tool at the correct angle to a planar abrasive surface and facilitating the application of uniform pressure along length of the cutting edge.